Lead free solder alloys containing Silver (Ag) (e.g., Tin (Sn)—Silver (Ag) and Tin (Sn)—Silver (Ag)—Copper (Cu) alloys) have been considered as one of the most promising replacements for solders containing Lead (Pb) for microelectronics applications. However, due to the rigidity of Sn—Ag and Sn—Ag—Cu alloys, compared with Pb-containing alloys, more component failures have been found in flip-chip and ball grid array (BGA) applications, where high stresses are developed. In addition, the high rigidity of these alloys have resulted in more high impact and drop induced interconnect failures for portable electronic devices, such as personal data assistants (PDA), notebook computers, etc.
Reducing the Ag concentration in Sn—Ag (B. Ebersberger et al., “Qualification of SnAg Solder Bumps for Lead-Free Flip Chip Applications”, Proceedings of IEEE Electronic Components and Technology Conference (ECTC), Las Vegas, 2004, pp. 683-691) and Sn—Ag—Cu alloys (Yoshiharu Kariya et al., “Effect of Silver Content on the Shear Fatigue Properties of Sn—Ag—Cu Flip-Chip Interconnects”, Journal of Electronic Materials, Vol. 33, No. 4, 2004, pp. 321-328) has resulted in more ductile and compliant solders for high stress conditions. However, Sn—Ag and Sn—Ag—Cu alloys containing reduced concentrations of Ag exhibit poor fatigue performance. In addition, the pasty range (i.e., the difference between the solidus and liquidus temperatures) of Sn—Ag and Sn—Ag—Cu alloys increases with decreasing Ag content and thus results in higher defect rates (see, S. K. Kang et al., “Microstructure and Mechanical Properties of Lead-Free Solders and Solder Joints Used in Microelectronic Applications”, IBM Journal of Research and Development, on-line publication: http:/researchweb.watson.ibm.com/journal/rd/494/kang.html).
In view of the foregoing, it would be desirable to provide a technique for increasing the compliance of Sn—Ag and Sn—Ag—Cu alloys without unduly increasing the pasty range and sacrificing fatigue resistance.